1. Field of the Invention
This invention relates to the structure of a microstrip antenna comprised of a radiator patch and feedline that are separated from a conductive ground plane by a space with a dielectric constant, hereinafter referred to as the dielectric space. More specifically, the invention relates to a microstrip antenna in which the dielectric space includes an air gap.
2. Description of the Related Art
The performance of an antenna is determined by several parameters, one of which is efficiency. For a microstrip antenna, "efficiency" is defined as the power radiated divided by the power received by the input to the antenna. A one-hundred percent efficient antenna has zero power loss between the received power input and the radiated power output. In the design and construction of microstrip antennas it is desirable to produce antennas having a relatively high efficiency rating, preferably in the range of 95 to 99 percent.
One factor in constructing a high efficiency microstrip antenna is minimizing power loss, which may be caused by several factors including dielectric loss. Dielectric loss is due to the imperfect behavior of bound charges, and exists whenever a dielectric material is located in a time varying electrical field. Moreover, because dielectric loss increases with operating frequency, the problem of dielectric loss is aggravated when operating at higher frequencies.
The extent of dielectric loss for a particular microstrip antenna is determined by, inter alia, the permittivity, .epsilon., expressed in units of farads/meter (F/m), of the dielectric space between the radiator and the ground plane which varies somewhat with the operating frequency of the antenna system. As a more convenient alternative to permittivity, the relative dielectric constant, .epsilon..sub.r, of the dielectric space may be used. The relative dielectric constant is defined by the equation: EQU .epsilon..sub.r =.epsilon./.epsilon..sub.o (i)
where .epsilon. is the permittivity of the dielectric space and .epsilon..sub.o is the permittivity of free space (8.854.times.10.sup.-12 F/m). It is apparent from this equation that free space, or air for most purposes, has a relative dielectric constant approximately equal to unity.
A dielectric material having a relative dielectric constant close to one is considered a "good" dielectric material--that is, the dielectric material exhibits low dielectric loss at the operating frequency of interest. When a dielectric material having a relative dielectric constant equal to unity is used, dielectric loss is effectively eliminated. Therefore, one method for maintaining high efficiency in a microstrip antenna system involves the use of a material having a low relative dielectric constant in the dielectric space between the radiator patch and the ground plane.
Furthermore, the use of a material with a lower relative dielectric constant permits the use of wider transmission lines that, in turn, reduce conductor losses and further improve the efficiency of the microstrip antenna.
The use of a material with a low dielectric constant, however, is not without drawbacks. For example, one dielectric material frequently used in microstrip antenna systems is Teflon fiberglass which has a typical relative dielectric constant of ranging from 2.1 to 2.6 in the radio-frequency (RF) range. Because Teflon fiberglass is expensive, however, the resultant cost of such a high-efficiency antenna system is prohibitive for many applications. Moreover, using a substrate material with a dielectric constant even as low as 2.1 may still result in significant dielectric loss at high operating frequencies.
Another suggested method to produce low dielectric loss microstrip antenna systems involves the use of a material having a honeycomb core, such as that sold under the mark HEXCEL HRP, to separate the radiator patch from the ground plane. A honeycomb core substrate material can have a dielectric constant as low as 1.09 at high frequencies, thereby reducing dielectric loss. The construction of an antenna system using a honeycomb core, however, is disadvantageous for several reasons. For example, both the honeycomb material and the glue required to bond the honeycomb material to the antenna elements are expensive. Additionally, the construction of an antenna utilizing a honeycomb substrate is burdensome due to the need to form the honeycomb into a narrow thickness and then carefully glue the honeycomb securely between the antenna radiator patch and the ground plane. Using this method will produce inaccurate and inefficient antenna systems unless very careful control of tolerances, glue-line thickness, and materials is maintained. Moreover, it is very expensive and technically difficult, if not impossible, to form the honeycomb material into a sufficiently thin and uniform height as required for high operating frequencies. Consequently, the expense and labor-intensity of this method makes it prohibitively expensive and burdensome for many applications.